Recently, to lower the cost and weight, a thin duct (the mean thickness is 1 mm or less) has been desired.
However, in a situation where a thin duct is molded by use of a sheet of thermoplastic resin in a melted state, since the sheet is thin, the sheet is quickly cooled and the welding strength of the parting line is lowered. Also, when the sheet is thin, the welding area of the parting line is small; hence, the welding strength of the parting line is lowered.
When the welding strength of the parting line is lowered, a crack easily takes place beginning at the parting line, and the welded sheets easily peel off from each other. Hence, when molding a thin duct, it is required to increase the welding strength of the parting line.
Incidentally, as technical literature filed prior to the present invention, there is literature (for example, patent document 1: Utility Model Laid-Open Ser. No. 5-87387) disclosing a technique to increase strength of a crashed section which is formed by crashing resin by metallic molds from the right and left sides along a parting line.